Presence of human-source fecal contamination in samples of produce, irrigation water, and hand rinse samples from northern Mexico

Over the last few decades, consumption of fresh and minimally processed produce has been associated with an increasing proportion of U.S. foodborne illnesses. The pathogens causing these outbreaks are primarily fecally derived. However, little is known about the relative importance of various risk factors in introducing fecal contamination to fresh produce, due in part to the fact that the food safety sector utilizes fecal indicator bacteria (FIB) as indicators of contamination. While FIB are useful indicators of fecal contamination, they cannot provide information about the source of contamination as they are found in both human and animal feces. Microbial source tracking (MST) is an emerging tool in environmental microbiology for identifying and quantifying the dominant source(s) of fecal contamination in surface waters. One promising MST method utilizes 16S rDNA sequences of Bacteroides, obligate anaerobic bacteria whose members are associated with different hosts. The purpose of this project was to apply Bacteroides MST assays to samples of produce rinses, irrigation water, and hand rinse samples collected over the course of one year from multiple farms (growing tomatoes, Jalapeno peppers, and cantaloupes) in Northern Mexico. Of 174 samples examined, 36% were positive for a universal Bacteroides marker (indicating general fecal contamination), with an average concentration of 5.5 genome equivalent copies per 100 ml. Of these, 5 samples (melon and hand rinse samples) were positive for a human-specific marker. No samples were positive for a bovine-specific marker. These results indicate that MST markers may be utilized to indicate the source of fecal contamination in food and environmental samples associated with fresh produce production. This provides for increased understanding of the relative importance of various risk factors in introducing pathogens to fresh produce, information essential for the development of effective measures to reduce the risk of produce contamination, thereby reducing risks to human health.